The crystallization kinetics of Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 were studied in an electrostatic levitation (ESL) apparatus. The measured critical cooling rate is 1.75 K/s. Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is the first bulk-metallic-glass-forming liquid that does not contain beryllium to be vitrified by purely radiative cooling in the ESL. Furthermore, the sluggish crystallization kinetics enable the determination of the time-temperature-transformation (TTT) diagram between the liquidus and the glass transition temperatures. The shortest time to reach crystallization in an isothermal experiment; i.e., the nose of the TTT diagram is 32 s. The nose of the TTT diagram is at 900 K and positioned about 200 K below the liquidus temperature

Hays, C. C.

Hyers, R. W.

Johnson, W. L.

Rathz, T. J.

Robinson, M. B.

Rogers, J. R.

Schroers, J.

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 11 10 SEPTEMBER 2001Vitrification and determination of the crystallization time scales
of the bulk-metallic-glass-forming liquid Zr 58.5Nb2.8Cu15.6Ni12.8Al10.3
C. C. Hays,a) J. Schroers, and W. L. Johnson
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125
T. J. Rathz
University of Alabama at Huntsville, Huntsville, Alabama 35899
R. W. Hyers, J. R. Rogers, and M. B. Robinson
NASA Marshall Space Flight Center, Huntsville, Alabama 35812
~Received 23 April 2001; accepted for publication 1 July 2001!
The crystallization kinetics of Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 were studied in an electrostatic levitation
~ESL! apparatus. The measured critical cooling rate is 1.75 K/s. Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is the
first bulk-metallic-glass-forming liquid that does not contain beryllium to be vitrified by purely
radiative cooling in the ESL. Furthermore, the sluggish crystallization kinetics enable the
determination of the time-temperature-transformation~TTT! diagram between the liquidus and the
glass transition temperatures. The shortest time to reach crystallization in an isothermal experiment;
i.e., the nose of the TTT diagram is 32 s. The nose of the TTT diagram is at 900 K and positioned
about 200 K below the liquidus temperature. ©2001 American Institute of Physics.
@DOI: 10.1063/1.1398605#rig
um
oy
r–
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ial
ting
ss
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25
ch
es
ma
f
K,The discovery of Zr-based bulk-metallic glasses has t
gered tremendous research activity. There are a limited n
ber of the multicomponent bulk-metallic-glass-forming all
systems; e.g., Zr–Ti–Cu–Ni–Be~Ref. 1!, Zr–Al–Cu–Ni
~Ref. 2!, Zr–Nb–Cu–Ni–Al ~Ref. 3! and Zr–Ti–Cu–Ni.4
The critical cooling rate required to vitrifyideal alloys in the
respective composition manifolds varies from 1.8 K/s in Z
Ti–Cu–Ni–Be, to 102 K/s in the quaternary Zr–Ti–Cu–N
system. Electrostatic levitation~ESL! and calorimetric meth-
ods have examined in detail the crystallization behavior
one of the beryllium containing bulk-metallic glasse
Zr41.2Ti13.8Cu12.5Ni10Be22.5.
5,6 Recently, a composition in the
Zr–Nb–Cu–Ni–Al system has been found that appears
have an excellent glass forming ability.7 Measurements on
the alloy Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 have shown that it can
be vitrified via conventional processing techniques; e.g.,
melting. With a critical casting thickness of greater than
cm, the expected critical cooling rate should be less than
K/s. This low cooling rate motivated our studies of this co
position in the ESL. The ESL provides an ideal platform f
the study of liquid metals in the metastable undercooled
uid region.8 This letter reports crystallization studies in th
undercooled liquid state of Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 ob-
tained via processing in the ESL. Constant cooling exp
ments are presented that determine the critical cooling
required to vitrify the material. Furthermore, the entire tim
temperature-transformation~TTT! diagram was determined
and allows for the interpretation of the crystallization kine
ics of this bulk glass-forming alloy under isothermal con
tions.
The Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 ESL specimens were
prepared from high purity elements via arc melting in
a!Author to whom all correspondence should be addressed; electronic
chays@hyperfine.caltech.edu1600003-6951/2001/79(11)/1605/3/$18.00
Downloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject to AIP-
-
f
,
to
rc
5
0
-
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-
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-
ultrahigh purity Ar atmosphere. Figure 1 shows a different
scanning calorimetry measurement conducted with a hea
rate of 20 K/min on a mg-sized ESL specimen. The gla
transition is observed atTg5674 K, followed by primary
crystallization atTx5776 K. The supercooled liquid regio
~SLR! interval is DT5Tx2Tg5102 K. ESL specimens
30–45 mg in mass, were cut from 25 g mass ingots. The
g ingots were vitrified on cooling in the arc furnace, ea
approximately 1.5 cm thick.
Figure 2 shows free and rate-controlled cooling curv
for a Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 ESL specimen. Curve 2~c!
il:
FIG. 1. Differential scanning calorimetry thermogram o
Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is shown. Glass transition temperature is 674
with primary crystallization at 776 K, when heated at 20 K/min.5 © 2001 American Institute of Physics
 license or copyright, see http://apl.aip.org/apl/copyright.jsp
t
s
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1606 Appl. Phys. Lett., Vol. 79, No. 11, 10 September 2001 Hays et al.shows the onset of crystallization at'880 K upon constan
cooling at 1.5 K/s. Crystallization is manifest by a recale
cence event; i.e., the reheating of the specimen due to
release of the heat of fusion. Cooling at 1.75 K/s@curve 2~b!#
is sufficient to circumvent detectable crystallization a
therefore vitrify the material. Curve 2~a! represents a free
cooling curve, and a corresponding maximum cooling rate
5–10 K/s in the critical temperature range near'800 K. In
order to prove that the sample of curve 2~b! was vitrified
upon cooling, following cooling the specimen was heated
'20 K/min. Crystallization was observed atTx'774 K, thus
verifying the glassy condition.
Prior to isothermal experiments, specimens were hea
to 1273 K, held for 60 s, and then cooled to the desi
isothermal temperature. The results of these isothermal c
tallization studies, performed between the liquidus a
glass transition temperatures are summarized in
Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 TTT diagram depicted in Fig. 3
which shows the time to reach crystallization for a giv
isothermal temperature. Strictly speaking, the radiative co
ing time to reach the plateau cannot be neglected in comp
son to the elapsed time at the plateau temperature. As s
the diagram determined is not a quantitative TTT diagra
On the other hand, the radiative cooling segment~'20 s! is
identical for all undercooling cycles so that the effect is s
tematic for all cycles. The data define a well-developed no
~Tnose, tnose! '~900 K, 32 s! which has also been observed
Zr41.2Ti13.8Cu12.5Ni10Be22.5 and Pd-based alloys.
9,10 The nose
of the TTT diagram is positioned about 200 K below t
liquidus temperature. Between 900–1000 K, and near 70
the liquid could be held for.30 min without recalescence
Subsequent cooling at 2 K/s vitrified the liquid~square data
points of Fig. 3!. Therefore, the square data points give
lower bound for the crystallization time. Above the nose
the curve there is relatively little scatter in the crystallizati
onset times. This behavior is different from what has be
observed in the bulk glass formers previously mention
FIG. 2. Cooling curves for Zr58.5Nb2.8Cu15.6Ni12.8Al10.3: ~a!—free cooling,
~b!—controlled cooling at 1.75 K/s, and~c!—controlled cooling at 1.5 K/s
are shown.Downloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject to AIP-
he
f
t
ed
d
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e
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ch,
.
-
e,
K,
f
n
d
~Refs. 9 and 10!. In Zr41.2Ti13.8Cu12.5Ni10Be22.5 and
Pd43Cu27Ni10P20, a large scatter in the time to reach crysta
lization above the nose of the TTT diagram was explained
a nucleation controlled crystallization mechanism.
In Fig. 3, at low temperatures just aboveTg , no differ-
ence in the time to reach crystallization were observed~dia-
mond data points of Fig. 3!, on cooling from the liquid state
or heating the vitrified material into the isothermal platea
However in Zr41.2Ti13.8Cu12.5Ni10Be22.5, as the isothermal
temperature increases towards the nose of the TTT diagr
a large asymmetry for crystallization, on heating versus co
ing, sets in.11 Thus, we performed rapid heating experimen
in the ESL on Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 specimens. Heat-
ing at rates up to 500 K/s still lead to crystallization sugges
ing an asymmetry of at least two orders of magnitude in t
critical cooling and critical heating rates. The explanation f
this asymmetry is given by the fact that in metallic system
the maximum in the nucleation rate is always at a low
temperature than the maximum in the crystal growth ra
This results in different critical heating and cooling rate
Nuclei formed upon cooling at the maximum in the nucle
ation rate experience a low growth rate, whereas nuc
formed at the nose of the TTT diagram upon heating a
exposed to the maximum growth rate. A similar behavior h
been observed in Zr41.2Ti13.8Cu12.5Ni10Be22.5.
12
The data of Fig. 3 represent a seminal example of t
TTT diagram for a bulk-metallic-glass-forming liquid. The
crystallization kinetics of Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 are
comparable to the beryllium containing glass
Zr41.2Ti13.8Cu12.5Ni10Be22.5, developed by Pekeret al.
1 The
nose of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 TTT diagram was ob-
served at 900 K and 48 s, which is located about 100
below the liquidus temperature. The TTT diagrams f
hese two alloys are quite similar, and indicate that t
main underlying physical mechanisms of crystallizatio
might be common in origin. The nose of the
FIG. 3. Experimental Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 the TTT diagram is
shown. Circles and squares denote data collected on cooling from the liq
state to an isothermal temperature. Diamonds depict data collected on h
ing from the amorphous state. license or copyright, see http://apl.aip.org/apl/copyright.jsp
r-
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av-
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1607Appl. Phys. Lett., Vol. 79, No. 11, 10 September 2001 Hays et al.Zr58.5Nb2.8Cu15.6Ni12.8Al10.3TTT diagram occurs at an unde
cooling roughly twice that of Zr41.2Ti13.8Cu12.5Ni10Be22.5,
200 K vs 100 K, respectively. The driving force for crysta
lization increases with decreasing temperature, while at
same time the atomic mobility decreases with temperat
The differing temperature dependence of these two contr
tions is manifest in the shape of the TTT diagram no
This suggests that for Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 either,
the free energy difference between the liquid and so
is smaller or the atomic mobility is greater than that
Zr41.2Ti13.8Cu12.5Ni10Be22.5 given the similarity of the curve
positions.
Several criteria are employed in the development of b
metallic glasses; e.g., compositions are close to deep eu
tics, and often exhibit large reduced glass transition temp
tures,Trg , whereTrg is the ratio of the glass transition tem
perature,Tg , to the liquidus temperature,Tliq . They exhibit
a high resistance to crystallization, manifest by large S
values. Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is the lowest melting
composition observed, to date, in this region of the com
sition manifold. Following Turnbull, glasses with critica
cooling rates ofRC'1 K/s whether polymeric or ceramic
often exhibit a value ofTrg'2/3.
13 From Fig. 2, the critical
cooling rate for Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is RC
51.75 K/s. This low value is contrasted by a modestTrg
5673 K/1103 K50.61. Again, from the Turnbull criteria
this low Trg50.61 is indicative of a poor glass-forming abi
ity; i.e., with RC51.75 K/s, we might expect aTrg closer to
2/3. Indeed, for Zr41.2Ti13.8Cu12.5Ni10Be22.5 Trg50.62
5623 K/1026 K. Thus, the Turnbull criteria cannot expla
the excellent glass-forming ability in these system
Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 also exhibits a large SLR, with
DT5102 K. However, this is not a necessary condition
good bulk glass formation. Recent work by Waniuket al. on
the beryllium-containing alloys first reported by Hayset al.
show that the critical cooling rate, and the SLR width, ha
an inverse trend.14,15The SLR width is indicative of what are
now known to be diffusion-controlled crystallization mech
nisms, as opposed to the nucleation controlled crystalliza
mechanisms active above the nose of the TTT diagram.
We have previously shown that a neighboring compo
tion of the investigated alloy, Zr57Nb5Cu15.4Ni12.6Al10, has
a TTT diagram which is comprised of two branches16
Indicating that two independent nucleation mechanisms li
the glass forming ability of this composition, which cou
not be vitrified on free cooling in the ESL. Indeed, a lar
scatter in the onset times to crystallization at high tempe
tures for this composition were observed, and related toDownloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject to AIPe
e.
u-
.
d
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k
ec-
a-
-
.
r
e
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i-
it
-
e
crossing of the high temperature branches. In fact,
Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 TTT diagram of Fig. 3 qualita-
tively represents what we referred to previously as the h
temperature branch of the Zr57Nb5Cu15.4Ni12.6Al10 TTT dia-
gram. Together, these results indicate that the solubility l
its, and resultant topological instability, of the crystallin
phases that compete with the glassy state may be contro
over quite narrow ranges in composition. The quantitat
aspects of these phenomena are the subject of upcoming
lications.
In conclusion, Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is the first
bulk-metallic-glass-forming liquid that does not contain b
ryllium to be vitrified by purely radiative Stefan–Boltzman
ooling in the electrostatic levitator. The critical cooling ra
required to vitrify this composition is 1.75 K/s. In addition
we have determined the TTT diagram for this alloy, and t
diagram provides alternate insights regarding the crystall
tion mechanisms active in bulk-glass-forming liquids.
The authors acknowledge the support of the Natio
Aeronautics and Space Administration~Grant No. NAG8-
1744! and the Department of Energy~Grant No. DEFG-03-
86ER45242!. Portions of this work were carried out in th
electrostatic levitation facility at the NASA Marshall Spac
Flight Center, Huntsville, AL. The authors thank Bob Turrin
of Caltech’s Graphics Arts Department.
1A. Peker and W. L. Johnson, Appl. Phys. Lett.63, 2342~1993!.
2T. Zhang, A. Inoue, and T. Masumoto, Mater. Trans., JIM32, 1005~1991!.
3X. H. Lin, Ph.D. dissertation, California Institute of Technology~1997!.
4X. H. Lin and W. L. Johnson, J. Appl. Phys.78, 6514~1995!.
5Y. J. Kim, R. Busch, W. L. Johnson, A. J. Rulison, and W. K. Rhim, Ap
Phys. Lett.68, 1057~1996!.
6T. A. Waniuk, R. Busch, A. Masuhr, and W. L. Johnson, Acta Mater.46,
5229 ~1998!.
7C. C. Hays, J. Schroers, U. Geyer, S. Bossuyt, N. Stein, and W
Johnson, J. Metastable and Nanocryst. Mater.8, 103 ~2000!.
8W.-K. Rhim, S. K. Chung, D. Barber, K. F. Man, G. Gutt, A. J. Ruliso
and R. E. Spjut, Rev. Sci. Instrum.64, 2961~1993!.
9J. Schroers, R. Busch, and W. L. Johnson, Appl. Phys. Lett.76, 2343
~2000!.
10J. Schroers, Y. Wu, R. Busch, and W. L. Johnson, Acta Mater.~in press!.
11J. Schroers, R. Busch, A. Masuhr, and W. L. Johnson, Phys. Rev. B60,
11855~1999!.
12J. Schroers, R. Busch, S. Bossuyt, and W. L. Johnson, Mat. Sci. E
14029~2000!.
13D. Turnbull, Contemp. Phys.10, 473 ~1969!.
14C. C. Hays, C. P. Kim, and W. L. Johnson, Appl. Phys. Lett.75, 1089
~1999!.
15T. A. Waniuk, J. Schroers, and W. L. Johnson, Appl. Phys. Lett.78, 1213
~2001!.
16C. C. Hays, J. Schroers, D. S. Lee, and W. L. Johnson, NASA Microgr
ity Workshop, Proc. Metals, Minerals & Materials Society, San Dieg
CA, 1998!. license or copyright, see http://apl.aip.org/apl/copyright.jsp


Vitrification and determination of the crystallization time scales of the bulk-metallic-glass-forming liquid Zr58.5Nb2.8Cu15.6Ni12.8Al10.3

APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 11 10 SEPTEMBER 2001Vitrification and determination of the crystallization time scales
of the bulk-metallic-glass-forming liquid Zr58.5Nb2.8Cu15.6Ni12.8Al10.3
C. C. Hays,a) J. Schroers, and W. L. Johnson
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125
T. J. Rathz
University of Alabama at Huntsville, Huntsville, Alabama 35899
R. W. Hyers, J. R. Rogers, and M. B. Robinson
NASA Marshall Space Flight Center, Huntsville, Alabama 35812
~Received 23 April 2001; accepted for publication 1 July 2001!
The crystallization kinetics of Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 were studied in an electrostatic levitation
~ESL! apparatus. The measured critical cooling rate is 1.75 K/s. Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is the
first bulk-metallic-glass-forming liquid that does not contain beryllium to be vitrified by purely
radiative cooling in the ESL. Furthermore, the sluggish crystallization kinetics enable the
determination of the time-temperature-transformation ~TTT! diagram between the liquidus and the
glass transition temperatures. The shortest time to reach crystallization in an isothermal experiment;
i.e., the nose of the TTT diagram is 32 s. The nose of the TTT diagram is at 900 K and positioned
about 200 K below the liquidus temperature. © 2001 American Institute of Physics.
@DOI: 10.1063/1.1398605#The discovery of Zr-based bulk-metallic glasses has trig-
gered tremendous research activity. There are a limited num-
ber of the multicomponent bulk-metallic-glass-forming alloy
systems; e.g., Zr–Ti–Cu–Ni–Be ~Ref. 1!, Zr–Al–Cu–Ni
~Ref. 2!, Zr–Nb–Cu–Ni–Al ~Ref. 3! and Zr–Ti–Cu–Ni.4
The critical cooling rate required to vitrify ideal alloys in the
respective composition manifolds varies from 1.8 K/s in Zr–
Ti–Cu–Ni–Be, to 102 K/s in the quaternary Zr–Ti–Cu–Ni
system. Electrostatic levitation ~ESL! and calorimetric meth-
ods have examined in detail the crystallization behavior of
one of the beryllium containing bulk-metallic glasses,
Zr41.2Ti13.8Cu12.5Ni10Be22.5.5,6 Recently, a composition in the
Zr–Nb–Cu–Ni–Al system has been found that appears to
have an excellent glass forming ability.7 Measurements on
the alloy Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 have shown that it can
be vitrified via conventional processing techniques; e.g., arc
melting. With a critical casting thickness of greater than 1.5
cm, the expected critical cooling rate should be less than 10
K/s. This low cooling rate motivated our studies of this com-
position in the ESL. The ESL provides an ideal platform for
the study of liquid metals in the metastable undercooled liq-
uid region.8 This letter reports crystallization studies in the
undercooled liquid state of Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 ob-
tained via processing in the ESL. Constant cooling experi-
ments are presented that determine the critical cooling rate
required to vitrify the material. Furthermore, the entire time-
temperature-transformation ~TTT! diagram was determined,
and allows for the interpretation of the crystallization kinet-
ics of this bulk glass-forming alloy under isothermal condi-
tions.
The Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 ESL specimens were
prepared from high purity elements via arc melting in an
a!Author to whom all correspondence should be addressed; electronic mail:
chays@hyperfine.caltech.edu1600003-6951/2001/79(11)/1605/3/$18.00
Downloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject toultrahigh purity Ar atmosphere. Figure 1 shows a differential
scanning calorimetry measurement conducted with a heating
rate of 20 K/min on a mg-sized ESL specimen. The glass
transition is observed at Tg5674 K, followed by primary
crystallization at Tx5776 K. The supercooled liquid region
~SLR! interval is DT5Tx2Tg5102 K. ESL specimens,
30–45 mg in mass, were cut from 25 g mass ingots. The 25
g ingots were vitrified on cooling in the arc furnace, each
approximately 1.5 cm thick.
Figure 2 shows free and rate-controlled cooling curves
for a Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 ESL specimen. Curve 2~c!
FIG. 1. Differential scanning calorimetry thermogram of
Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is shown. Glass transition temperature is 674 K,
with primary crystallization at 776 K, when heated at 20 K/min.5 © 2001 American Institute of Physics
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
1606 Appl. Phys. Lett., Vol. 79, No. 11, 10 September 2001 Hays et al.shows the onset of crystallization at ’880 K upon constant
cooling at 1.5 K/s. Crystallization is manifest by a recales-
cence event; i.e., the reheating of the specimen due to the
release of the heat of fusion. Cooling at 1.75 K/s @curve 2~b!#
is sufficient to circumvent detectable crystallization and
therefore vitrify the material. Curve 2~a! represents a free
cooling curve, and a corresponding maximum cooling rate of
5–10 K/s in the critical temperature range near ’800 K. In
order to prove that the sample of curve 2~b! was vitrified
upon cooling, following cooling the specimen was heated at
’20 K/min. Crystallization was observed at Tx’774 K, thus
verifying the glassy condition.
Prior to isothermal experiments, specimens were heated
to 1273 K, held for 60 s, and then cooled to the desired
isothermal temperature. The results of these isothermal crys-
tallization studies, performed between the liquidus and
glass transition temperatures are summarized in the
Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 TTT diagram depicted in Fig. 3,
which shows the time to reach crystallization for a given
isothermal temperature. Strictly speaking, the radiative cool-
ing time to reach the plateau cannot be neglected in compari-
son to the elapsed time at the plateau temperature. As such,
the diagram determined is not a quantitative TTT diagram.
On the other hand, the radiative cooling segment ~’20 s! is
identical for all undercooling cycles so that the effect is sys-
tematic for all cycles. The data define a well-developed nose,
~Tnose, tnose! ’~900 K, 32 s! which has also been observed in
Zr41.2Ti13.8Cu12.5Ni10Be22.5 and Pd-based alloys.9,10 The nose
of the TTT diagram is positioned about 200 K below the
liquidus temperature. Between 900–1000 K, and near 700 K,
the liquid could be held for .30 min without recalescence.
Subsequent cooling at 2 K/s vitrified the liquid ~square data
points of Fig. 3!. Therefore, the square data points give a
lower bound for the crystallization time. Above the nose of
the curve there is relatively little scatter in the crystallization
onset times. This behavior is different from what has been
observed in the bulk glass formers previously mentioned
FIG. 2. Cooling curves for Zr58.5Nb2.8Cu15.6Ni12.8Al10.3: ~a!—free cooling,
~b!—controlled cooling at 1.75 K/s, and ~c!—controlled cooling at 1.5 K/s
are shown.Downloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject to~Refs. 9 and 10!. In Zr41.2Ti13.8Cu12.5Ni10Be22.5 and
Pd43Cu27Ni10P20, a large scatter in the time to reach crystal-
lization above the nose of the TTT diagram was explained by
a nucleation controlled crystallization mechanism.
In Fig. 3, at low temperatures just above Tg , no differ-
ence in the time to reach crystallization were observed ~dia-
mond data points of Fig. 3!, on cooling from the liquid state
or heating the vitrified material into the isothermal plateau.
However in Zr41.2Ti13.8Cu12.5Ni10Be22.5, as the isothermal
temperature increases towards the nose of the TTT diagram,
a large asymmetry for crystallization, on heating versus cool-
ing, sets in.11 Thus, we performed rapid heating experiments
in the ESL on Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 specimens. Heat-
ing at rates up to 500 K/s still lead to crystallization suggest-
ing an asymmetry of at least two orders of magnitude in the
critical cooling and critical heating rates. The explanation for
this asymmetry is given by the fact that in metallic systems,
the maximum in the nucleation rate is always at a lower
temperature than the maximum in the crystal growth rate.
This results in different critical heating and cooling rates.
Nuclei formed upon cooling at the maximum in the nucle-
ation rate experience a low growth rate, whereas nuclei
formed at the nose of the TTT diagram upon heating are
exposed to the maximum growth rate. A similar behavior has
been observed in Zr41.2Ti13.8Cu12.5Ni10Be22.5.12
The data of Fig. 3 represent a seminal example of the
TTT diagram for a bulk-metallic-glass-forming liquid. The
crystallization kinetics of Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 are
comparable to the beryllium containing glass,
Zr41.2Ti13.8Cu12.5Ni10Be22.5, developed by Peker et al.1 The
nose of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 TTT diagram was ob-
served at 900 K and 48 s, which is located about 100 K
below the liquidus temperature. The TTT diagrams for
hese two alloys are quite similar, and indicate that the
main underlying physical mechanisms of crystallization
might be common in origin. The nose of the
FIG. 3. Experimental Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 the TTT diagram is
shown. Circles and squares denote data collected on cooling from the liquid
state to an isothermal temperature. Diamonds depict data collected on heat-
ing from the amorphous state. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
1607Appl. Phys. Lett., Vol. 79, No. 11, 10 September 2001 Hays et al.Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 TTT diagram occurs at an under-
cooling roughly twice that of Zr41.2Ti13.8Cu12.5Ni10Be22.5,
200 K vs 100 K, respectively. The driving force for crystal-
lization increases with decreasing temperature, while at the
same time the atomic mobility decreases with temperature.
The differing temperature dependence of these two contribu-
tions is manifest in the shape of the TTT diagram nose.
This suggests that for Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 either,
the free energy difference between the liquid and solid
is smaller or the atomic mobility is greater than that of
Zr41.2Ti13.8Cu12.5Ni10Be22.5 given the similarity of the curve
positions.
Several criteria are employed in the development of bulk
metallic glasses; e.g., compositions are close to deep eutec-
tics, and often exhibit large reduced glass transition tempera-
tures, T rg , where T rg is the ratio of the glass transition tem-
perature, Tg , to the liquidus temperature, T liq . They exhibit
a high resistance to crystallization, manifest by large SLR
values. Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is the lowest melting
composition observed, to date, in this region of the compo-
sition manifold. Following Turnbull, glasses with critical
cooling rates of RC’1 K/s whether polymeric or ceramic,
often exhibit a value of T rg’2/3.13 From Fig. 2, the critical
cooling rate for Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is RC
51.75 K/s. This low value is contrasted by a modest T rg
5673 K/1103 K50.61. Again, from the Turnbull criteria,
this low T rg50.61 is indicative of a poor glass-forming abil-
ity; i.e., with RC51.75 K/s, we might expect a T rg closer to
2/3. Indeed, for Zr41.2Ti13.8Cu12.5Ni10Be22.5 T rg50.62
5623 K/1026 K. Thus, the Turnbull criteria cannot explain
the excellent glass-forming ability in these systems.
Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 also exhibits a large SLR, with
DT5102 K. However, this is not a necessary condition for
good bulk glass formation. Recent work by Waniuk et al. on
the beryllium-containing alloys first reported by Hays et al.
show that the critical cooling rate, and the SLR width, have
an inverse trend.14,15 The SLR width is indicative of what are
now known to be diffusion-controlled crystallization mecha-
nisms, as opposed to the nucleation controlled crystallization
mechanisms active above the nose of the TTT diagram.
We have previously shown that a neighboring composi-
tion of the investigated alloy, Zr57Nb5Cu15.4Ni12.6Al10, has
a TTT diagram which is comprised of two branches.16
Indicating that two independent nucleation mechanisms limit
the glass forming ability of this composition, which could
not be vitrified on free cooling in the ESL. Indeed, a large
scatter in the onset times to crystallization at high tempera-
tures for this composition were observed, and related to theDownloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject tocrossing of the high temperature branches. In fact, the
Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 TTT diagram of Fig. 3 qualita-
tively represents what we referred to previously as the high
temperature branch of the Zr57Nb5Cu15.4Ni12.6Al10 TTT dia-
gram. Together, these results indicate that the solubility lim-
its, and resultant topological instability, of the crystalline
phases that compete with the glassy state may be controlled
over quite narrow ranges in composition. The quantitative
aspects of these phenomena are the subject of upcoming pub-
lications.
In conclusion, Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is the first
bulk-metallic-glass-forming liquid that does not contain be-
ryllium to be vitrified by purely radiative Stefan–Boltzmann
cooling in the electrostatic levitator. The critical cooling rate
required to vitrify this composition is 1.75 K/s. In addition,
we have determined the TTT diagram for this alloy, and this
diagram provides alternate insights regarding the crystalliza-
tion mechanisms active in bulk-glass-forming liquids.
The authors acknowledge the support of the National
Aeronautics and Space Administration ~Grant No. NAG8-
1744! and the Department of Energy ~Grant No. DEFG-03-
86ER45242!. Portions of this work were carried out in the
electrostatic levitation facility at the NASA Marshall Space
Flight Center, Huntsville, AL. The authors thank Bob Turring
of Caltech’s Graphics Arts Department.
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Vitrification and determination of the crystallization time scales of the bulk-metallic-glass-forming liquid Zr58.5Nb2.8Cu15.6Ni12.8Al10.3

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